1. Field of the Invention
The present invention relates to an organic-inorganic composite insulating material which can be preferably used in various fine electronic elements and a method of producing same. The present invention also relates to a field-effect transistor comprising the organic-inorganic composite insulating material for electronic element.
2. Description of the Related Art
In recent years, organic semiconductor devices have been extensively studied aiming at the development of new application different from that of related art silicon semiconductor devices. Among these devices, organic thin film transistors (OTFT) formed on a flexible substrate such as plastic sheet have become of particular interest. Examples of the preparation of these organic thin film transistors have been reported. Thus, further enhancement of performance have been expected (see “Advanced Material”, 2002, No. 2, page 99).
Of course, the enhancement of the performance of semiconductor layer is essential for the enhancement of the performance of PTFT. In addition, the selection of the structure and member through which the performance of semiconductor layer can be sufficiently drawn, particularly the enhancement of the performance of the insulated gate layer is indispensable. Further, since the preparation of the silicon oxide gate insulating film requires the highest temperature heat treatment at the silicon semiconductor process, it is required that the insulated gate film be prepared at a temperature as low as not higher than the heat-resisting temperature of plastic substrate (resin substrate).
On the other hand, in order to make the use of the cost advantage of plastic substrate, a simple process for the preparation of elements involving spreading such as ink ejection and spin coating or a process for the preparation of elements requiring no high temperature heat treatment is required.
The requirements for insulated gate film in OTFT on a flexible substrate are (1) no damage such as crack despite the bending of the substrate (flexibility), (2) producibility by spin coating or like method, (3) producibility at low temperatures, (4) chemical resistance, (5) high dielectric constant, and (6) high insulation resistance (electric strength).
Most of the materials of insulated gate film for OTFT on flexible substrate which have heretofore been reported are inorganic materials such as inorganic oxide and organic polymers. However, these materials can difficultly meet all the aforementioned requirements.
Insulating films made of these inorganic materials exhibit a high dielectric constant but lack flexibility. On the other hand, insulating films made of organic polymers are flexible. However, insulating films made of phenolic resin exhibit a relatively high dielectric constant but lack dielectric strength. Insulating films made of polyimide exhibit a strong chemical resistance but require a high temperature heat treatment to obtain its chemical resistance. Thus, insulating films made of organic polymers can meet some of these requirements but some other requirements. It is extremely difficult for these insulating films to meet all these requirements.
Japanese Patent No. 3,515,507 proposes that an organic polymer and an inorganic material be mixed to provide the resulting insulating film with flexibility and high dielectric constant. In accordance with this reference, a powder obtained by mechanically grinding a ferroelectric material such as barium titanate is dispersed in an organic polymer to compensate the dielectric constant of the resulting insulated gate film and hence lower the gate voltage required for the operation of transistor.
However, when this method is used, the thickness of the insulating film is limited to the size of the inorganic material thus ground. Further, since a solid material is dispersed in an organic polymer solution, an uneven dispersion is formed, possibly causing the generation of local electric field and concurrent dielectric breakdown during the operation of transistor. Moreover, since the inorganic material is merely present in the organic polymer and thus doesn't compensate the chemical resistance of the insulating film, the resulting insulating film cannot be subjected to any processes involving the use of solvents.
JP-A-2003-338551 discloses a technique of forming a thin ceramics film as an insulating film on the surface of silicon wafer by a sol-gel method allowing a low temperature treatment. In accordance with this technique, the resulting thin ceramics film can be prevented from being cracked, making it possible to efficiently produce electronic parts having a high reliability. However, the thin ceramics film is an insulating film made of an inorganic material that can be applied to silicon wafer, which is nonflexible and hard, but cannot be applied to flexible substrates.
On the other hand, in an attempt to obtain a uniformly dispersed organic-inorganic mixture system as an ordinary material technique, it has been practiced to prepare a composite film from a mixture of a solution of metal alkoxide which is a precursor of inorganic oxide and an organic polymer solution by a sol-gel method. In this case, it is expected that as the dispersion of organic polymer is made more on monomolecular level, the thermal stability of the organic polymer is enhanced more.
Referring to the mutual action of metal alkoxide with organic polymer, it has been reported that when a polymer or molecules capable of making hydrogen bond such as hydroxyl group or electrostatic mutual action are present in a metal alkoxide solution, a sol-gel polycondensation proceeds selectively on the surface of the compound to form a dried gel (“Advanced Material”, 2002, No. 2, page 99).
However, the mixing of metal alkoxide and organic polymer has been limited to the doping of organic dye molecules as optical material with a high dispersion, the use of an organic polymer as a mold for the purpose of finally burning away the organic polymer to form a porous oxide film or the academic studies of mutual action of the two materials.